The male fertility genes of Drosophila melanogaster will be examined in the vermillion, v, and maroon-like, mal, regions of the X-chromosome. Male-sterile, ms, mutations and lethal mutations induced in these regions will be mapped by conventional genetic methods. The ms mutations will be positioned relative to the lethal loci in each region. The frequency of ms loci relative to the number of lethal loci will thus be assessed for each region. The sperm of ms mutants will be studied by cytological methods to determine the effect of the genetic lesion and the developmental stage at which the lethal condition is evident will be determined for each lethal locus. Tests for allelism will be performed for lethal and ms mutations. We plan to determine 1) if each male fertility gene is essential for a specific process in sperm development of function; 2) if male fertility genes, in some cases, facilitate a vital function; and 3) if genes for male fertility are concentrated in the proximal region of the X-chromosome.